Prosthetic devices have been used for hundreds, if not thousands, of years as aids in solutions to certain types of medical problems. In recent bio-medical technology, such prosthetic devices have become far more sophisticated, not only as to scope of problems to which they are applicable, but also in their technological sophistication. Current prosthetic devices rely heavily on electronic or electromechanical activity and, accordingly, need electrical power sources. Because the people requiring the need for such prosthetic devices must carry on an active life requiring mobility, external power sources are impractical. In addition, leads through the skin pose special surgical and bacteriostatic problems. These problems have not yet been solved.
Accordingly, for many years there have been proposed implanted energy sources. Batteries, otherwise known as energy cells, have long been proposed. However, significant problems remain with respect to batteries intended to power such prosthetic devices as cardiac pacemakers, neurostimulators, and physiological monitors. They have also shown to be entirely inadequate for more sophisticated devices on the horizon such as heart pumps and limb activators.
The possibility of utilizing chemical species normally available within the body as reactants for an implantable energy source has been explored for some years. These include the ion concentration cells (1), biochemical fuel cells (2-8), and bio-galvanic cells (9-14). However, all of these types of cells have serious problems which have prevented their practical utilization to any significant extent.
For example, ion concentration cells are limited by concentration differences between different body fluids and, accordingly, cannot produce sufficient or reliable power (15).
Regarding biochemical fuel cells, by taking advantage of biological homeostasis, fuel cells which utilize endogenous compounds could function in principle for a lifetime. Theoretically, the cell volume could be minimized because no reactant storage is required.
Generally, existing energy sources such as energy cells (batteries) must be implanted remotely from the power-requiring device or prosthesis because of their weight and size. The connection between the prosthesis and the battery is provided by electrical leads. However, there has been a frequently reported occurrence of lead failure in existing implantable energy sources (3). Thus, the proposal for miniature, lightweight energy cells for implantation in direct association with the energy dissipating device (the prosthesis) at its site of action would tend to overcome lead failure problems.
One of the problems with biochemical fuel cells is that the energy generated in chemical reactions of such implantable fuel cells is usually on the order of only 10-100 microwatt (.mu.-watt). However, such a level is sufficient to power the low-energy demand devices such as the electronic pacemaker (3). Even so, it would not be suitable for more power-hungry devices.
Another problem which has arisen in regard to biochemical fuel cells is their dependence on body fluids which renders them a power supply that is subject to variations in output due to fluctuations in the body's supply of ions, fuels, and oxidants. Since such cells and their electrodes are exposed to numerous components of body fluids, they are difficult to satisfactorily model in vitro, or to translate to in vivo operation. Such cells are called bio-auto fuel cells since they obtain both oxidant and reductant (fuel) from the body. Our previously reported work (3,4,17) on such bio-auto cells demonstrates a full knowledge of in vivo parameters is a necessary prerequisite to design of a satisfactorily functioning cell. These in vivo parameters such as ion, fuel, and oxygen concentration, osmotic pressure, and the like are either unknown or may vary with biological rhythms and implantation sites.
Regarding bio-galvanic cells, previous work since 1971 has been done on an encapsulated bio-galvanic cell (16). The bio-galvanic cell consists of a corroding metal anode and a cathode which utilizes oxygen from the body fluids. Some cells are designed with a "sealed" anode compartment so they do not release toxic products to the body. Other bio-galvanic cells have a corroding metal anode in contact with body tissues. These cells have been subject to uneven and unpredictable electrodissolution of the anode, which has compounded the problem of lead breakage. This longevity is limited by the amount of self-contained reactive metal, and is a function of the weight and dimensions of the cell. If the cell ruptures, toxic products are released in the body.
Accordingly, there is a need for still another approach to providing suitable energy sources which can be satisfactorily encapsulated for implantation in association with biological tissues, that is, are biologically acceptable and non-reactive. These cells must be small and provide sufficient power either by themselves or as a group of cells in a "battery" assembly, as to provide sufficient power for present day and future proposed prostheses. These cells should not be site specific, that is, requiring intimate knowledge of the special biochemical parameters of implantation sites and highly variable biological rhythms of the many different types and conditions of patients into which they are to be implanted. Further, they should be free from lead breakage problem and not be subject to lifetime limitations inherent in original charge of corrodible fuel.
The present invention solves these needs in the form of an intermittently refuelable encapsulated bio-oxidant fuel cell having a self-contained reductant and utilizing endogenous supply of oxygen present in all tissues and which is capable of being refueled and evacuated at infrequent intervals, thereby not requiring an initial charge of fuel for its entire life. In the event of rupture, no toxic fuel or by-products are released into the body.